Incoherent light-emitting device apparatus for driving vertical laser cavity

ABSTRACT

A laser emitting apparatus includes a substrate having on one side an incoherent light-emitting device having a light-emitting layer wherein an electric field is applied across the light-emitting layer to produce light which is transmitted out of the incoherent light-emitting device through an optically transparent layer into a vertical laser cavity structure disposed to receive light transmitted from the incoherent light-emitting device and produce laser light.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation-in-part of commonly-assigned U.S. patentapplication Ser. No. 09/832,759, filed Apr. 11, 2001, U.S. Pat. No.6,658,037 entitled “Incoherent Light-Emitting Device Apparatus forDriving Vertical Laser Cavity” by Keith B. Kahen et al.

FIELD OF THE INVENTION

The present invention relates to the field of light-emitting devices, inparticular, to organic-based solid-state lasers.

BACKGROUND OF THE INVENTION

Over the past number of years, there has been increasing interest inmaking organic-based solid-state lasers. The lasing material has beeneither polymeric or small molecule and a number of different resonantcavity structures were employed, such as, microcavity (Kozlov et al.,U.S. Pat. No. 6,160,828), waveguide, ring microlasers, and distributedfeedback (see also, for instance, G. Kranzelbinder et al., Rep. Prog.Phys. 63, 729 (2000) and M. Diaz-Garcia et al., U.S. Pat. No.5,881,083). A problem with all of these structures is that in order toachieve lasing it was necessary to excite the cavities by opticalpumping using another laser source. It is much preferred to electricallypump the laser cavities since this generally results in more compact andeasier to modulate structures.

A main barrier to achieving electrically-pumped organic lasers is thesmall carrier mobility of organic material, which is typically on theorder of 10⁻⁵ cm²/(V-s). This low carrier mobility results in a numberof problems. Devices with low carrier mobilities are typicallyrestricted to using thin layers in order to avoid large voltage dropsand ohmic heating. These thin layers result in the lasing modepenetrating into the lossy cathode and anode, which causes a largeincrease in the lasing threshold (V. G. Kozlov et al., J. Appl. Phys.84, 4096 (1998)). Since electron-hole recombination in organic materialsis governed by Langevin recombination (whose rate scales as the carriermobility), low carrier mobilities result in orders of magnitude morecharge carriers than single excitons; one of the consequences of this isthat charge-induced (polaron) absorption can become a significant lossmechanism (N. Tessler et al., Appl. Phys. Lett. 74, 2764 (1999)).Assuming laser devices have a 5% internal quantum efficiency, using thelowest reported lasing threshold to date of ˜100 W/cm² (M. Berggren etal., Nature 389, 466 (1997)), and ignoring the above mentioned lossmechanisms, would put a lower limit on the electrically-pumped lasingthreshold of 1000 A/cm². Including these loss mechanisms would place thelasing threshold well above 1000 A/cm², which to date is the highestreported current density, which can be supported by organic devices (N.Tessler, Adv. Mater. 10, 64 (1998)).

One way to avoid these difficulties is to use crystalline organicmaterial instead of amorphous organic material as the lasing media. Oneof the advantages of organic-based lasers is that since the material istypically amorphous, the devices can be formed inexpensively and theycan be grown on any type of substrate. The single-crystal organic-laserapproach obviates both of these advantages.

A few others have suggested pumping the organic laser cavity withlight-emitting diodes (LED's), either inorganic (M. D. McGehee et al.,Appl. Phys. Lett. 72, 1536 (1998)) or organic (Berggren et al., U.S.Pat. No. 5,881,089). McGehee et al. (M. D. McGehee et al., Appl. Phys.Lett. 72, 1536 (1998)) state that they needed to lower their thresholdsby at least an order of magnitude to attempt laser pumping using anInGaN LED. Berggren et al. propose making an all organic unitary laserwhere one section of the device (the organic LED part) provides theincoherent radiation, while the adjacent section (the laser cavity)provides optical down conversion, gain and optical feedback. Berggren etal. state that the lasing cavity should be either a waveguide withfacets, a distributed-feedback waveguide cavity, adistributed-Bragg-reflector waveguide cavity, or a photonic-latticecavity. Berggren et al. only showed data for the organic light-emittingdiode (OLED) section of the device (its current-voltage andvoltage-luminance characteristics). With respect to the device's lasingcharacteristics, their only comment was that it produced coherentradiation at ˜620 nm. Since Berggren et al. never gave any additionaldetails with respect to the device's lasing operation, it is difficultto determine if the device lased as a result of excitation from the OLEDsection of the device. Consequently, to the best of our knowledge, therehave not been any documented cases of laser cavities excited byincoherent light sources.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improvedarrangement for using light produced by an incoherent light-emittingdevice as input to a vertical laser cavity structure for producing laserlight wherein the vertical laser cavity structure is on a common side ofa substrate with the incoherent light emitting device.

This object is achieved by an laser emitting apparatus, comprising:

(a) an optically transparent layer;

(b) an incoherent light emitting device including;

-   -   (i) a first transparent electrode located on one side of the        optically transparent layer;    -   (ii) a light emissive layer adjacent the first electrode to        produce a pump beam light which is transmitted out of the        incoherent light-emitting device through the first transparent        electrode and the optically transparent layer;    -   (iii) a second electrode adjacent the light emissive layer;

(c) a vertical laser cavity structure located on the other side of theoptically transparent layer and disposed to receive the pump beam lighttransmitted from the incoherent light-emitting device through theoptically transparent layer, such structure including:

-   -   (i) first means for receiving light from the incoherent        light-emitting device and being mainly transmissive or        reflective over predetermined ranges of wavelengths;    -   (ii) an organic active layer for receiving light from the        incoherent light-emitting device and from the first        light-receiving means and for producing laser light; and    -   (iii) second means for reflecting light from the organic active        layer back into the organic active layer, wherein a combination        of the two means transmits the laser light; and

(d) a substrate located adjacent to either the second electrode or thesecond means.

It has been found that a vertical laser cavity is particularly suitablefor receiving incoherent light from an incoherent light-emitting deviceand, when integrated with the incoherent light emitter on one side of asubstrate, permits the integration of other system elements on the otherside of the substrate. It is a further advantage of the presentinvention to use a vertical laser cavity design incorporating highreflectance dielectric multi-layer mirrors for both the top and bottomreflectors and to have the active material composed of small-molecularweight organic material. As a result the laser cavity has a very lowthreshold. This is a consequence of: 1) the small active volume; 2) theusage of very low-loss, high-reflectivity dielectric mirrors; 3) thelasing medium being composed of small-molecular weight organic materialswhich can be deposited very uniformly over the bottom dielectric stack;and 4) the lasing medium being composed of a host organic material(absorbs the incoherent radiation) and a small volume percentage dopantorganic material (emits the laser light) which results in a high quantumefficiency and low scattering/absorption loss. It was also found, quiteunexpectedly, that the threshold power density dropped by orders ofmagnitude as a result of significantly increasing the cross-sectionalarea and pulse width (on the order of microseconds) of the pump lightbeam. The consequence of the very low threshold for the vertical lasercavity is that it is unnecessary to use high-power density devices(focused laser light) in order to cause the cavity to lase. As a result,low power density devices, such as unfocused OLED radiation, aresufficient light sources to be used for pumping the laser cavities.Combining an organic-based laser cavity with an OLED pump source on oneside of a single substrate results in an inexpensive and versatile lasersource whose light output can be tuned over a large wavelength range.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-section of a prior art organic solid-statelaser device;

FIG. 2 is a schematic cross-section view of one embodiment of an organicsolid-state laser apparatus made in accordance with the presentinvention, the apparatus being composed of two sections: one section isan electrically-driven OLED device which emits incoherent radiation;while, the second section is a low threshold vertical laser cavity whichabsorbs the OLED radiation and emits laser light at a longer wavelength;

FIG. 3 is a schematic cross-section view of an alternative embodiment ofthe present invention;

FIG. 4 is a log-log plot of the dependence of output power on the inputexcitation power for an exemplary embodiment of a vertical laser cavitydiscussed in Example 1;

FIG. 5 is a high resolution spectrum of the lasing transition emittedfrom the vertical laser cavity of Example 1;

FIG. 6 is a log-log plot of the dependence of output power on the inputexcitation power for two exemplary embodiments of a vertical lasercavity discussed in Example 2. Cavities A and C have active layerthicknesses of 195 and 780 nm, respectively;

FIG. 7 is a spectrum of the output intensity emitted by the (verticallaser) Cavity A structure discussed in Example 2. The light is collectedin the normal viewing direction;

FIG. 8 is a log-log plot of the dependence of output power on the inputexcitation power for the (vertical laser) Cavity B (390 nm active layerthickness) structure discussed in Example 2;

FIG. 9 is a spectrum of the relative output intensity of the OLED devicediscussed in Example 3. The OLED was driven at 20 mA/cm² and theradiation was collected in the normal viewing direction;

FIG. 10 is a high resolution spectrum of the lasing transition emittedfrom the OLED-pumped vertical laser cavity (Cavity A) of Example 3;

FIG. 11 is a log-log plot of the dependence of output power on the drivecurrent for an electrically-driven organic solid-state laser device ofExample 3, where the vertical laser cavity structure is Cavity B and theOLED is being driven using pulse widths of 2 μs and 8 μs;

FIG. 12 is a schematic cross-section view of the organic solid-statelaser apparatus embodiment of FIG. 2; and

FIG. 13 is a schematic cross-section view of the organic solid-statelaser apparatus embodiment of FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

In order to more fully appreciate the construction and performance ofthe two-section electrically-driven organic solid-state laser apparatus,a prior art organic laser-cavity device 100 of FIG. 1 will be described.

In the prior art shown in FIG. 1, an organic laser-cavity device 100 hasa transparent substrate 105 on which is formed a mirror layer 110. Thetransparent substrate 105 can be glass or quartz, while the mirror layer110 is a distributed Bragg reflector (DBR) dielectric mirror stack. DBRmirrors consist of λ/4 thick dielectric layers, where λ represents thecenter wavelength of the DBR mirror reflective stop-band, and the stackalternates layers of high and low refractive index. The reflectance ofthe DBR mirror is typically in excess of 99%. Typical dielectricmaterials used in forming the DBR mirror is SiO₂ for the low-indexmaterial and TiO₂ or Ta₂O₅ for the high-index material. An organicactive layer 115 is formed over the mirror layer 110. The organic activelayer 115 can be composed of either small-molecular weight organicmaterial or conjugated polymer organic material. The small-molecularweight organic material is typically deposited by high-vacuum thermalevaporation, while the conjugated polymers are usually formed by spincasting. Over the organic active layer 115 is deposited a metal 120 bythermal evaporation. Typical metals are silver or aluminum, which havereflectivities in excess of 90%. To get the device 100 to lase, theorganic active layer 115 is optically pumped by an incident light beam125. Because of the requirement that the incident light beam 125 delivera high optical energy density to the active layer 115, it is typical touse a laser as the incident light source in combination with anappropriate lens. The organic active layer 115 absorbs the incident pumpbeam and then emits some fraction of that energy as light of a longerwavelength. Some of the long-wavelength light is emitted as unwantedspontaneous emission, while another fraction is emitted as stimulatedemission 130 which is directed normal to the plane of the layers andexits the device through the bottom mirror layer 110 and the transparentsubstrate 105.

The high lasing threshold of the organic laser-cavity device 100 is theresult of a number of factors. Using a metal layer as one of thereflectors results in approximately 10% of the laser light lost duringeach round trip inside of the laser cavity. In addition, having metalswithin ˜150 nm of the active layer can result in significant quenchingof the active material's fluorescence (K. B. Kahen, Appl. Phys. Lett.78, 1649 (2001)). It is also typical to use conjugated polymers as theactive material. Since these materials are deposited by spin casting, itis difficult to achieve good thickness uniformity over the surface ofthe active layer. These thickness non-uniformities would result indifferences in the round trip phase as a function of lateral position onthe device. As a result, destructive interference can occur which wouldlead to higher thresholds. An additional issue for conjugated polymeractive layers (which don't use host-dopant combinations) is that at thelasing wavelength there is still significant absorption from the activematerial.

FIG. 2 is a schematic cross-section of an organic solid-state laserapparatus 200 according to one embodiment of the present invention. Itis composed of two sections. The first section 201 is a vertical lasercavity which differs from the prior art in that an optically transparentlayer 205 is located between an organic light emitting diode (OLED) 231incoherent light source and the vertical laser cavity 201, and bothreflectors 210 and 220 are DBR mirrors and the active layer 215 isformed from organics which employ a host-dopant material system. In thepreferred embodiment, optically transparent layer 205 is an opticallytransparent insulating planarization layer compatible with an OLEDincoherent light source, for example silicon dioxide; however, it can beany optically-flat layer compatible with an OLED incoherent light sourceand upon which a DBR mirror can be grown. The DBR mirror 210 isdeposited on the optically transparent layer 205. It is preferred to begrown by conventional sputtering or electron-beam (e-beam) depositionsince it is important to get accurate thicknesses for the dielectriclayers. The bottom DBR mirror 210 is composed of alternating high andlow refractive index dielectric layers, such that, at the wavelength forthe laser light 230 its reflectivity is greater than 99.9% and ittransmits greater than 90% of the OLED light 225. DBR mirror 210 iscomposed of λ/4-thick alternating high and low refractive indexdielectric layers in order to get a high-reflectance at the lasingwavelength, λ₁; additional alternating high and low refractive indexdielectric layers are also deposited such that there results a broadtransmission maximum for the OLED light 225. Over the DBR mirror 210 isdeposited the organic active layer 215, which can be formed byconventional high-vacuum (10⁻⁶ Torr) thermal vapor deposition or by spincasting from solution. In order to obtain low thresholds, it ispreferred that the thickness of the organic active layer 215 be integermultiples of λ/2, where λ is the lasing wavelength. The lowestthresholds are obtained for the integer multiple being either 1 or 2.The organic active layer 215 includes host and dopant organic molecules.It is preferred that the organic molecules be of small-molecular weightsince currently they can be deposited more uniformly. The host materialsused in the present invention are selected from any materials that havesufficient absorption of the OLED light 225 and are able to transfer alarge percentage of their excitation energy to a dopant material viaForster energy transfer. Those skilled in the art are familiar with theconcept of Forster energy transfer, which involves a radiationlesstransfer of energy between the host and dopant molecules. An example ofa useful host-dopant combination for red-emitting lasers is aluminumtris(8-hydroxyquinoline) (Alq) as the host and4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran(DCJTB) as the red-emitting dopant. A DBR mirror 220 is deposited overthe organic active layer 215. It is also deposited by conventionale-beam deposition; however, this time it is preferred that during thedeposition process the temperature of the organic stay below 75° C. Thetop DBR mirror 220 is composed of alternating high and low refractiveindex dielectric layers, such that, at the wavelength for the laserlight 230 its reflectivity is greater than 98% and it reflects greaterthan 90% of the OLED light 225. Consequently, besides depositing theλ/4-thick alternating high and low refractive index dielectric layers(where λ is chosen near the desired lasing wavelength), additionalalternating high and low refractive index dielectric layers aredeposited such that there results a broad reflection maximum for theOLED light 225. In particular, it is only necessary to reflect thatportion of the OLED light 225 which is absorbed by the organic activelayer 215 host material.

The second OLED 231 section of the organic solid-state laser emittingapparatus 200 is one or more electrically-driven organic light-emittingdiode devices which produce incoherent light within a predeterminedportion of the spectrum. For an example of an OLED device, see commonlyassigned U.S. Pat. No. 6,172,459 to Hung et al., and the referencescited therein, the disclosures of which are incorporated by reference.

The organic light-emitting diode 231 is formed adjacent to, andpreferably on, a substrate 235 a on which is formed an electrode 240,for example an anode. The substrate 235 a can be any material suitablefor the construction of OLED devices as are described in the art, forexample glass or quartz, and the electrode 240 can be composed of indiumtin oxide (ITO) or high work function metals (such as Au). The electrodecan be deposited by evaporation (thermal or e-beam) or sputtering. Anorganic hole-transport layer 245 is formed over the electrode 240, anorganic light-emitting layer 250 is formed over the organichole-transport layer 245, and an organic electron-transport layer 255 isformed over the organic light-emitting layer 250. As an example forthese three layers, a useful structure includes a diamine layer, suchas, 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPB) for the organichole-transport layer 245, undoped 9,10-Bis(2-naphthalenyl)anthracene(ADN) as the organic light-emitting layer 250, and Alq as the organicelectron-transport layer 255. These organics are typically prepared byhigh-vacuum thermal evaporation. Their preferred thicknesses are 40-250nm for the NPB, 10-50 nm for the ADN, and 10-200 nm for the Alq. Asecond transparent electrode 260 (a cathode) is formed over the organicelectron-transport layer 255, and of a material selected to have a workfunction less than 4.0 eV. A suitable transparent electrode 260 is MgAg,where the Mg—Ag volume ratio is 10:1. It can be formed by conventionalthermal vapor deposition. Since the cathode needs to be transparent inorder to enable the OLED light 225 to pass into the vertical lasercavity 201, the preferred thickness of a metal-based cathode is lessthan 15 nm. An optically transparent layer 205 is formed over thecathode and the vertical cavity laser 201 formed upon the opticallytransparent layer 205. Additional layers, as are known in the art, canbe included in the OLED structure, for example hole-injection andelectron-injection layers. As is well understood in the art, a voltage Vcan be applied across the electrodes to provide the necessary electricfield for causing the organic light-emitting layer 250 to produce thepump beam light, which is transmitted out of the organic light-emittingdiode device. The voltage V can be continuous or in the form of pulses.

Under typical bias conditions, electrons (negative-charge carriers) willbe injected from the transparent electrode 260 into the organicelectron-transport layer 255, and holes (positive charge carriers) willbe injected from the electrode 240 into the organic hole-transport layer245. Electrons and holes are transported through the correspondingorganic layers 255 and 245 and into the organic light-emitting layer250. In the organic light-emitting layer 250 the electrons and holesmainly recombine near the junction between the organic hole-transportlayer 245 and the organic light-emitting layer 250. The resultingrecombination results in light emission from the organic light-emittinglayer 250. Of the light generated in the organic light-emitting layer250, approximately 50% is directly emitted in the direction of thesubstrate 235 a while the other 50% is emitted directly toward thetransparent electrode 260. The transparent electrode 260 is partiallytransparent and allows the light to pass through the opticallytransparent layer 205 to optically pump the vertical laser cavity. Theelectrode 240 and/or the underlying substrate can be made reflective sothat the portion of the light emitted toward the substrate can bereflected out of the device to pass through the optically transparentlayer 205 as well.

After exiting the organic light-emitting diode 231, the OLED light 225enters the vertical laser cavity 201 through the bottom DBR mirror 210.As a result of the bottom DBR mirror design, the majority of that lightpasses into the organic active layer 215. By construction, the organicactive layer 215 absorbs some fraction of the OLED light 225. Of thefraction of light which did not get absorbed (for cases where theorganic active layer's absorption length is too small), the remainingfraction of OLED light 225 enters the top DBR mirror 220, whereby alarge fraction of the light is back-reflected into the organic activelayer 215 for a second pass. During the second pass, an additionalfraction of the OLED light 225 is absorbed by the organic active layer215. Via the Forster energy transfer mechanism, the light energyabsorbed by the host is non-radiatively transferred to the dopantmolecules. It is preferred that the dopant molecule has a high quantumefficiency for emission since that results in the majority of thenon-radiatively transferred energy being re-emitted as longer wavelengthlight. For example, with ADN as the OLED light emitter material, Alq asthe active layer host, and DCJTB as the active layer dopant, the emittedOLED light is blue, Alq mainly absorbs in the blue, while DCJTB emits inthe red. The vertical laser cavity 201 is designed to be a high-Q cavityfor red light, especially for wavelengths where the top and bottom DBRmirrors (210 and 220) have their highest reflectivities. Those skilledin the art are familiar with the concept that lasing occurs at aparticular wavelength which has the highest net gain. At thatwavelength, the laser light 230 reflects many times between the top andbottom DBR mirrors (210 and 220) prior to being emitted mainly throughthe top DBR mirror 220 (since by design the mirror loss of the bottomDBR mirror 210 is much lower than that of the top DBR mirror 220).

In this embodiment, the vertical laser cavity 201 and the organiclight-emitting diode 231 have been combined into an integrated deviceformed on one side of a single substrate with the organic light-emittingdiode 231 located on the substrate 235 a and the vertical laser cavity201 located above the organic light-emitting diode 201 and separatedfrom it by the optically transparent layer 205.

Referring to FIG. 3, in an alternative embodiment of the presentinvention, the substrate 235 b is transparent and is located adjacent tothe vertical laser cavity 201, and preferably the vertical laser cavity201 is formed upon the transparent substrate 235 b, so that light isemitted through the transparent substrate 235 b. The transparentelectrode is shown as an anode.

In either the embodiment shown in FIG. 2 or in FIG. 3, the organic solidstate laser apparatus 200 is formed upon the substrate 235 a ortransparent substrate 235 b, respectively, that also includes passive-or active-matrix circuitry providing means to operate and provide powerto the organic solid state laser apparatus 200. Such circuitry is wellknown in the art and found, for example, in OLED and LCD displaydevices. In this manner, an array of independently controlled laseremitters is formed on a common substrate 235 a or transparent substrate235 b. Moreover, as is well known, the lasers can be made to emit lightof different frequencies, by forming the laser cavities with varyingdimensions and by varying the active layers host and dopant combination.The organic light emitting diode 231 can emit different colors throughthe use of different emissive materials. It is also possible to createthe optically transparent layer 205 as a part of the DBR mirror 210 onone side of the vertical laser cavity 201.

Referring to FIG. 12, the organic solid state laser apparatus 200 of thepresent invention as shown in the embodiment of FIG. 2 can be formed byfirst providing the substrate 235 a, forming any desired circuitry 234(including a reflective electrode 240, for example an anode) on thesubstrate 235 a using photolithographic and deposition methods wellknown in the integrated circuit industry. Layers of organic material arethen formed upon the electrode 240 in a top-emitter configuration as isknown in the art to form an organic light emitting diode 231. Atransparent electrode 260 (e.g., a cathode) is formed above the layersof organic material. The transparent electrode 260 can be a commonelectrode for the OLED emitting elements. Above the transparentelectrode 260 a optically transparent layer 205 is formed. Thisoptically transparent layer 205 can protect the transparent electrode260 as well as forming a layer upon which the elements of the verticallaser cavity 201 can be formed. The vertical laser cavity 201 is thenformed upon the optically transparent layer 205. The organic solid statelaser apparatus 200 is encapsulated with an additional layer or glasscover (not shown) and is bonded to the substrate using, for example,techniques useful with OLED displays. The electrode 240, the organiclight emitting diode 231, and the transparent electrode 260 may extendover the circuitry 234 when provided with additional insulating andplanarization layers (not shown) over the circuitry 234 as is known inthe art. The electrode 240 and the organic light emitting diode 231 maybe pixilated as necessary to form independently controllable lightemitters.

Referring to FIG. 13, in the alternative embodiment of FIG. 3, thetransparent substrate 235 b is first provided and any desired circuitry234 formed upon the transparent substrate 235 b as described aboveincluding a portion 260 a of the transparent electrode 260. (Thisportion 260 a need not be transparent and can be highly conductive sinceno light is emitted through it.) The elements of the vertical lasercavity 201 are then formed over the transparent substrate 235 b adjacentto the portion of the transparent electrode 260 a. The opticallytransparent layer 205 is formed above the vertical laser cavity elementsbut not over the adjacent electrode, forming a via. An additional layerof transparent conductive material is deposited upon the opticallytransparent layer 205 and over the adjacent portion 260 a of thetransparent electrode 260, completing the transparent electrode 260. Thelayers of organic materials including the organic light-emitting diode231 are then deposited, followed by the second, reflective electrode 240in a bottom emitter configuration. The second electrode 240 can be acommon electrode while the transparent electrode 260 and the organiclight emitting diode 231 may be pixilated as necessary to formindependently controllable light emitters. Materials for transparent andreflective electrodes are well known in the art. The organic solid statelaser apparatus 200 is then encapsulated and sealed as described above.

The following examples are presented for a further understanding of thepresent invention and are not to be construed as limitations thereon.

EXAMPLE 1

In order to determine the general lasing characteristics of the organicsolid-state laser device described in FIGS. 2 and 3, a vertical lasercavity structure was formed on a pre-cleaned 4-inch Si substrate. Overthe substrate was deposited by conventional e-beam deposition the bottomDBR mirror, which was composed of alternating high and low refractiveindex layers of Ta₂O₅ and SiO₂, respectively. The resulting mirror had agreater than 99% reflectance stop band between 600 and 720 nm, where atthe center wavelength of 660 nm its reflectivity was greater than99.999%. On the top of the bottom DBR mirror was deposited by highvacuum thermal evaporation the active layer composed of 200 nm of Alqdoped with 1% of DCJTB. Lastly, the top DBR mirror was deposited by lowtemperature e-beam deposition, such that the measured temperature of theSi substrate was kept below 72° C. It was composed of alternating highand low refractive index layers of TiO₂ and SiO₂, respectively. Theresulting mirror had a greater than 99% reflectance stop band between665 and 775 nm, where at the center wavelength of 720 nm itsreflectivity was greater than 99.9%. The active layer thickness waschosen, such that, the vertical laser cavity structure would have alasing wavelength, λ₁, of approximately 690 nm. More specifically, theactive layer thickness was chosen to be λ₁/2n, where n (=1.691) is themeasured index of refraction of the active material at 690 nm.

The vertical laser cavity structure was optically pumped using a blueGaN laser diode (λ=419 nm). The diode was driven by a function generator(HP) at 8 V with a 4 KHz repetition rate so as to produce 50 ns pulses.It was determined that at 8 V the diode puts outs ˜30 mW cw. Using a 160mm lens, the pump beam was focused normally onto the surface of thevertical laser cavity structure to a measured spot size of 62 μm. Theenergy of the pulses were varied through use of calibrated neutraldensity filters. The emission spectrum in the cavity normal direction(with approximately a 16° full angle acceptance cone) was frequencydispersed using a double monochromator (Spex) and detected by a cooledphotomultiplier tube (Hamamatsu).

FIG. 4 shows a log-log plot of the dependence of output power on theinput excitation power for both the laser transition at 684 nm and aspontaneous emission peak at 626 nm. The spontaneous emission peak at626 nm is due to the sharp fall-off in the reflectance of the top DBRmirror beyond the reflectance stop band (665-775 nm); thus, at 626 nmthe measured reflectance of the top stack is approximately 3%. As can beseen from the figure, only the lasing transition shows a kink in thepower plot for low excitation energies, while both transitions roll-offat high power densities due to a quenching phenomenon. Even moresignificant is that the threshold pump power density is approximately0.06 W/cm² (or 3 nJ/cm²), which is orders of magnitude smaller than thelowest thresholds reported to date in the literature (M. Berggren etal., Nature 389, 466 (1997) and T. Granlund et al., Chem. Phys. Lett.288, 879 (1998)). Lastly, the figure shows that the slope of the lasingtransition is greater than that of the spontaneous emission feature(0.91 compared to 0.75). Besides the kink in the power plot and thelarger slope for the lasing transition, additional evidence for lasingis given in FIG. 5, which shows a high-resolution spectrum of the lasingpeak near 684 nm. Since the FWHM of the peak is 0.4 nm, which is at theresolution limit of the monochromator, the lasing transition is at leastthis narrow. On the other hand, the measured FWHM of the spontaneousemission peak at 626 nm is 7 nm. Both peaks were measured at an inputpower of 0.6 W/cm² (an order of magnitude above the lasing threshold).

This example demonstrates that by employing vertical laser cavitystructures of our design, extremely low lasing thresholds can beobtained. It is the result of these low thresholds which will enable usto excite these laser cavities using incoherent light sources.

EXAMPLE 2

In this example vertical laser cavity structures analogous to thosedescribed in Example 1 will be discussed. Three cavities (with Sisubstrates) were constructed which nominally were designed to lase at660 nm. Cavity A had an active layer thickness of λ₁/2n (=195 nm),cavity B had an active layer thickness of λ₁/n (=390 nm), and cavity Chad an active layer thickness of 2λ₁/n (=780 nm). All three activelayers were composed of Alq doped with 1% of DCJTB. The top and bottomDBR mirrors were the same in all three cases and were constructed asfollows. The bottom DBR mirror was composed of alternating high and lowrefractive index layers of TiO₂ and SiO₂, respectively. The resultingmirror had a greater than 99% reflectance stop band between 580 and 750nm, where at the center wavelength of 665 nm its reflectivity wasgreater than 99.999%. In addition, the mirror had a broad reflectancemaximum centered at 445 nm, whose peak reflectance was greater than 92%.The top DBR mirror was also composed of alternating high and lowrefractive index layers of TiO₂ and SiO₂, respectively. The resultingmirror had a greater than 99% reflectance stop band between 625 and 745nm, where at the center wavelength of 685 nm its reflectivity wasgreater than 99.9%. In addition, the mirror had a broad transmissionmaximum centered at 445 nm, where the average transmittance was greaterthan 97%.

FIG. 6 shows a log-log plot of the dependence of output power on theinput excitation power for Cavity A (195 nm thick active layer) andCavity C (780 nm thick active layer), where again the excitation sourceis the GaN laser diode operating at 8V with a 5 KHz repetition rate andproducing 50 ns wide pulses. For Cavities A and C the lasing transitionsoccurred at 671.5 and 681 nm, respectively. The figure shows that thelasing transition becomes more pronounced for microcavities containinglarger active layer thicknesses. This microcavity observation waspreviously noted by Yokoyama et al. (H. Yokoyama et al., Appl. Phys.Lett. 58, 2598 (1991)) and is additional evidence that the verticalcavities are producing laser light. Also in agreement with the resultsof Yokoyama et al., the figure shows that the threshold power increasesas the active layer thickness goes from 195 nm to 780 nm (0.07 W/cm² to0.22 W/cm²). It should be noted that for Cavity C the threshold powerdensity was taken at the end of the high slope transition region; morethan likely the threshold occurs somewhere within the transition region.

For Cavity A, FIG. 7 shows a spectrum of the lasing transition at 671.5nm and a spontaneous emission peak at approximately 594 nm for an inputexcitation power of 7 W/cm² (two orders of magnitude above the lasingthreshold). Again the spontaneous emission peak appears due to the sharpfall-off in the reflectance of the top DBR mirror beyond its reflectancestop band (625-745 nm). As before, its FWHM is ˜7 nm. The figure showsthat the cavity's emission spectrum is completely dominated by the highgain, spectrally narrow laser transition.

FIG. 8 shows a log-log plot of the dependence of output power on theinput excitation power for Cavity B (390 nm thick active layer) forthree different input beam excitation conditions, to be called CavitiesB1, B2, and B3. Cavities B1-B3 refer to three different laser pump beamconditions: B1) 10 KHz repetition rate, 10 ns pulse width, and a 62 μmcircular beam spot; B2) 4 KHz repetition rate, 50 ns pulse width, and a2.5 mm wide square spot; and B3) 4 KHz repetition rate, 2 μs pulsewidth, and a 2.5 mm wide square spot. All three of them were excitedwith the GaN laser diode operating at voltages of 8, 8, and 7 V,respectively (7 V corresponds to ˜22 mW cw). All three cavities hadlasing wavelengths near 666 nm. The general trend from the figure isthat the threshold power density decreases both due to an increase inthe beam spot size and pulse width (as qualified below). Comparing theresults from FIG. 8 with that from FIG. 6, it is seen that the thresholdpower density result for Cavity B1 (0.14 W/cm²) is in line with thosefor Cavities A (0.07 W/cm²) and C (0.22 W/cm²). As a result, thereappears to be no (or at best a small) impact of going from a 10 ns to 50ns pump beam pulse width. Comparing Cavities B1 and B2 it can be seenthat the threshold drops by a factor 35 as a result of increasing theinput beam spot area by a factor of 2000. It is important to note thatthe below and above threshold log-log power-curve slopes are veryanalogous for these two conditions: 0.68 and 0.96 for Cavity B1 and 0.71and 0.91 for Cavity B2 (note that for Cavity A the corresponding slopeswere 0.76 and 0.92). Next comparing Cavities B2 and B3, it can be seenthat increasing the pulse width from 50 ns to 2 μs results in a furtherdrop in the threshold power density by a factor of 10 to 0.0004 W/cm².Remarkably the above threshold log-log power-curve slope remains nearlyunchanged at 0.92, while the below threshold slope markedly increases to1.24 (to become a lasing transition region). Combining both of theseresults (comparing Cavities B1 and B3), a greater than two orders ofmagnitude drop in the threshold power density occurs upon increasing thespot size from 3×10⁻⁵ to 0.063 cm² and increasing the pump beam widthfrom 50 ns to 2 μs. Finally, for Cavity B3 its power conversionefficiency (laser power out divided by pump beam power in) wasdetermined to be approximately 0.06% at an order of magnitude above thethreshold input power density. Consequently, 1.67 mW of blue input poweris required to produce 1 μW of red output power. By lowering the top DBRmirror reflectivity and providing some lateral confinement to the lasingmode, it should be possible to raise considerably the power conversionefficiency number.

This example demonstrates, quite unexpectedly, that through increases inthe pump beam pulse width and beam size, one can obtain significantdrops in the lasing threshold power density, which will further enableOLED driven (electrically-pumped) laser cavities.

EXAMPLE 3

This is an example of the embodiment given in FIG. 2, where theincoherent light output 225 from an organic light-emitting diode 231 isused to drive a vertical laser cavity 201. Cavities A and B, asdescribed in Example 2, were used as the vertical laser cavitystructures, while the OLED device was constructed as follows:

a) a 85 nm thick transparent anode of ITO-coated glass wasultrasonicated in a commercial detergent, rinsed in deionized water,degreased in toluene vapor, and contacted by a strong oxidizing agent;

b) a 150 nm thick NPB hole-transport layer was deposited over the ITOanode by conventional thermal vapor deposition;

c) a 30 nm thick ADN light-emitting layer was deposited over the NPBlayer by conventional thermal vapor deposition;

d) a 20 nm thick Alq electron-transport layer was deposited over thelight-emitting layer by conventional thermal vapor deposition;

e) a 100 nm thick Mg-Ag cathode was deposited over theelectron-transport layer by conventional thermal vapor deposition. TheMg to Ag volume ratio was 10:1.

The OLED device was driven electrically by a function generator (HP) inseries with an amplifier (Avtech) which could deliver to high impedanceloads from 0 to 24 V. In order to monitor the current delivered to theOLED device, a 27 ohm resistor was put in series with the OLED and itsvoltage was measured by a 100 MHz digital oscilloscope (Textronics). Apair of 60 mm lenses was used to 1:1 image the output from the OLEDpixel (3 mm×3 mm) normally on to the surface of the vertical lasercavity structures.

At a cw drive current of 20 mA/cm², the OLED device had a measuredradiance (collected in the normal viewing direction) of 1.46 W/(Sr-m²).FIG. 9 shows a spectral plot of the OLED's relative radiance. As can beseen from the figure, the peak radiance is at 448 nm. Since the Alqabsorption coefficient rapidly begins to drop at 450 nm, only a portionof the OLED output is absorbed by the laser cavity.

FIG. 10 shows a spectrum of the laser output from Cavity A driven by theincoherent light produced by the OLED device. Results are given at anOLED current just above threshold. The spectrum was measured by amonochromator with slightly less resolution (0.55 nm instead of 0.40 nm)than the one used for FIG. 5. As a result, the FWHM of the laser line is0.55 nm. It should be noted also that for Cavity A driven by 50 ns widelaser pulses focused to a 62 μm spot, the monochromator used for FIG. 10also measured a spectral width of 0.55 nm. Consequently, the narrownessof the lasing transition is unaffected by going to a wider (in time andarea) and incoherent input pump beam.

FIG. 11 shows a log-log plot of the dependence of the laser output poweron the OLED current density for the electrically-pumped organicsolid-state laser device (Cavity B is the vertical laser cavitystructure). Results are given for two different current pulse widths of2 μs and 8 μs, where for both cases the repetition rate is 4 KHz. Eachof the log-log power curves shows three linear sections (and twocorresponding kinks). The high slope of the first linear portion of thelog-log power plot is due to the non-linear effects of the RC timeconstant of the OLED device which is on the order of 1 μs for small OLEDdrive currents. For the 2 μs pulsed device, the middle and upper linearsections have slopes of 1.22 and 1.04, respectively, which are verysimilar to the 1.24 and 0.92 slopes reported above in Example 2 withreference to Cavity B3 (same laser cavity but driven with a laser inputbeam whose beam shape and pulse width nearly match that of the OLEDoutput). The similarity between the power slopes for the laser-drivenand OLED-driven vertical laser cavities shows that the power curvecharacteristics don't depend on either the coherency or spectralcharacteristics of the pump beam power source. For the 8 μs pulseddevice, FIG. 11 shows that it behaves similarly to the 2 μs pulseddevice, with the linear slopes of the middle and upper sections being1.13 and 0.98, respectively. The figure also shows that the thresholdcurrents are approximately 0.5 and 0.3 A/cm² for the 2 and 8 μs pulseddevices, respectively.

As disclosed in both of the embodiments (shown in FIGS. 2 and 3respectively), a vertical laser cavity is particularly suitable forreceiving incoherent light from the incoherent light-emitting devicewherein both the laser cavity and the incoherent light emitting deviceare constructed upon a single, common side of a substrate. This permitsthe integration of other system elements on the other side of thesubstrate.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

PARTS LIST

-   100 organic laser-cavity device-   105 transparent substrate-   110 mirror layer-   115 organic active layer-   120 metal-   125 incident light beam-   130 stimulated emission-   200 organic solid-state laser apparatus-   201 vertical laser cavity-   205 optically transparent layer-   210 DBR mirror-   215 active layer-   220 DBR mirror-   225 OLED light-   230 laser light-   231 organic light emitting diode (OLED)-   234 circuitry-   235 a substrate-   235 b transparent substrate-   240 electrode-   245 organic hole-transport layer-   250 organic light-emitting layer-   255 organic electron-transport layer-   260 transparent electrode-   260 a portion of the transparent electrode

1. A laser emitting apparatus, comprising: a) an optically transparentlayer; b) an incoherent light emitting device including: i) a firsttransparent electrode located on one side of the optically transparentlayer; ii) a light emissive layer adjacent the first electrode toproduce a pump beam light which is transmitted out of the incoherentlight-emitting device through the first transparent electrode and theoptically transparent layer; iii) a second electrode adjacent the lightemissive layer; c) a vertical laser cavity structure located on theother side of the optically transparent layer and disposed to receivethe pump beam light transmitted from the incoherent light-emittingdevice through the optically transparent layer, such structureincluding: i) first means for receiving light from the incoherentlight-emitting device and being mainly transmissive or reflective overpredetermined ranges of wavelengths; ii) an organic active layer forreceiving light from the incoherent light-emitting device and from thefirst light-receiving means and for producing laser light; and iii)second means for reflecting light from the organic active layer backinto the organic active layer, wherein a combination of the two meanstransmits the laser light; and d) a substrate located adjacent to eitherthe second electrode or the second means.
 2. The laser emittingapparatus of claim 1 wherein the second electrode is located on thesubstrate.
 3. The laser emitting apparatus of claim 2 wherein the secondelectrode is reflective.
 4. The laser emitting apparatus of claim 2wherein the substrate is reflective.
 5. The laser emitting apparatus ofclaim 2 wherein the laser light is emitted through the second means forreflecting light from the organic active layer back into the organicactive layer.
 6. The laser emitting apparatus of claim 1 wherein thesecond means for reflecting light from the organic active layer backinto the organic active layer is located on the substrate.
 7. The laseremitting apparatus of claim 6 wherein the substrate is transparent. 8.The laser emitting apparatus of claim 6 wherein the laser light isemitted through the substrate.
 9. The laser emitting apparatus of claim1 wherein the vertical laser cavity structure is selected to producelaser light in a predetermined range of the spectrum.
 10. The laseremitting apparatus of claim 1 wherein the optically transparent layer isa part of the first means for receiving light from the incoherentlight-emitting device.
 11. The laser emitting apparatus of claim 1further including active-matrix control circuitry located upon thesubstrate for controlling the operation of the laser emitting apparatus.12. The laser emitting apparatus of claim 1 further includingpassive-matrix control circuitry located upon the substrate forcontrolling the operation of the laser emitting apparatus.
 13. The laseremitting apparatus of claim 1 wherein the incoherent light-emittingdevice is a top-emitter OLED device.
 14. The laser emitting apparatus ofclaim 1 wherein the incoherent light-emitting device is a bottom-emitterOLED device.
 15. The laser emitting apparatus of claim 1 furthercomprising a plurality of laser emitters located on a common substrate.16. The laser emitting apparatus of claim 1 wherein the laser lightemitted is red, green, or blue.
 17. A laser emitting apparatus,comprising: a) an optically transparent layer; b) an incoherent lightemitting device including: i) a first transparent electrode located onone side of the optically transparent layer; ii) a light emissive layeradjacent the first electrode to produce a pump beam light which istransmitted out of the incoherent light-emitting device through thefirst transparent electrode and the optically transparent layer; iii) asecond electrode adjacent the light emissive layer; c) a vertical lasercavity structure located on the other side of the optically transparentlayer and disposed to receive the pump beam light transmitted from theincoherent light-emitting device through the optically transparentlayer, such structure including: i) a first DBR mirror for receiving andtransmitting light from the incoherent light-emitting device and beingreflective to laser light over a predetermined range of wavelengths; ii)an organic active layer for receiving transmitted light from the firstDBR mirror and for producing laser light; and iii) a second DBR mirrorfor reflecting transmitted incoherent light and laser light from theorganic active layer back into the organic active layer and fortransmitting laser light; and d) a substrate located adjacent to eitherthe second electrode or the second DBR mirror.
 18. The laser emittingapparatus of claim 17 wherein the second electrode is located on thesubstrate.
 19. The laser emitting apparatus of claim 17 wherein thesecond electrode is reflective.
 20. The laser emitting apparatus ofclaim 17 wherein the substrate is reflective.
 21. The laser emittingapparatus of claim 17 wherein the second means for reflecting light fromthe organic active layer back into the organic active layer is locatedon the substrate.
 22. The laser emitting apparatus of claim 21 whereinthe substrate is transparent.
 23. The laser emitting apparatus of claim21 wherein the laser light is semitted through the substrate.
 24. Thelaser emitting apparatus of claim 17 wherein the vertical laser cavitystructure is selected to produce laser light in a predetermined range ofthe spectrum.
 25. A laser emitting apparatus, comprising: a) anoptically transparent layer; b) an incoherent light emitting deviceincluding: i) a first transparent electrode located on one side of theoptically transparent layer; ii) a light emissive layer adjacent thefirst electrode to produce a pump beam light which is transmitted out ofthe incoherent light-emitting device through the first transparentelectrode and the optically transparent layer; iii) a second electrodeadjacent the light emissive layer; c) a vertical laser cavity structuredisposed to receive a pump beam light transmitted from the incoherentlight-emitting device through the optically transparent layer, suchstructure including: i) a first DBR mirror for receiving andtransmitting light from the incoherent light-emitting device and beingreflective to laser light over a predetermined range of wavelengths; ii)an organic active layer for receiving transmitted incoherent light fromthe first DBR mirror and for producing laser light; and iii) a secondDBR mirror for reflecting transmitted incoherent light and laser lightfrom the organic active layer back into the organic active layer, thefirst DBR mirror being adapted to transmit laser light; and d) asubstrate located adjacent to either the second electrode or the secondDBR mirror.
 26. The laser emitting apparatus of claim 25 wherein thesecond electrode is located on the substrate.
 27. The laser emittingapparatus of claim 26 wherein the second electrode is reflective. 28.The laser emitting apparatus of claim 27 wherein the substrate isreflective.
 29. The laser emitting apparatus of claim 28 wherein thelaser light is emitted through the second means for reflecting lightfrom the organic active layer back into the organic active layer. 30.The laser emitting apparatus of claim 25 wherein the second means forreflecting light from the organic active layer back into the organicactive layer is located on the substrate.
 31. The laser emittingapparatus of claim 30 wherein the substrate is transparent.
 32. Thelaser emitting apparatus of claim 30 wherein the laser light is emittedthrough the substrate.
 33. The laser emitting apparatus of claim 25wherein the vertical laser cavity structure is selected to produce laserlight in a predetermined range of the spectrum.